This application is based on applications Nos. Hei 10-083453, Hei 10-133043, and Hei 10-271225 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method for production of a metal oxide thin film and an arrayed metal oxide functional device having a high quality capable of being used for various purposes, and more particularly to a method for production of a thin film and a functional device, made of a metal oxide which is useful as a material for optical modulators, ferroelectric memories, dielectric films for ICs, optical shutters, actuators, and micromachines that are widely used in the field of communication devices.
2. Description of the Related Art
In a general production method of a metal oxide thin film according to a sol/gel method, a sol is applied onto a substrate material and is dried to give a gel, followed by a heat treatment or radiation of high energy ultraviolet rays such as an excimer laser beam to decompose and remove organic substances contained in the starting material so as to give the metal oxide film. At this time, if the organic substances in the film are strongly bonded to metal ions, a heat treatment temperature for removing the organic substances must be high. If the speed of raising the temperature for the heat treatment is increased, the densification caused by the heat treatment on the surface film proceeds at a higher speed than that in the inside of the film, so that the organic substances are enclosed in the film. As a result, voids are formed after the removal of the organic substances by a final heat treatment at a high temperature, so that a dense film cannot be formed. This problem can be solved to some extent by lowering the speed of raising the temperature at the heat treatment, but not satisfactorily.
U.S. Pat. No. 5,453,294 discloses production of a PZT thin film by the sol/gel method in which a starting material solution is applied onto a substrate and then, after thermal decomposition at 150 to 250xc2x0 C., 250 to 350xc2x0 C., or 450 to 550xc2x0 C., crystallization is carried out at 550 to 800xc2x0 C. This method may control the orientation of crystals by using different temperatures for the thermal decomposition and the crystallization.
However, since the thermal decomposition and the removal of the organic substances before the crystallization are insufficient, the density of the obtained film is considerably poor.
Japanese Patent Application Laid-open No. Hei 6-5946 discloses production of a PZT thin film by the sol/gel method in which a process of applying a starting material solution onto a substrate, drying the substrate at 200 to 300xc2x0 C., and calcining the substrate at 650xc2x0 C. in an oxygen atmosphere is repeated to obtain a given thickness, and then calcining the substrate in an ozone atmosphere to prevent formation of oxygen holes in the crystal structure. However, since the organic substances cannot be removed to a sufficient extent simply by adjusting the condition at the thermal decomposition step, so that the density of the film is poor.
Japanese Patent Application Publication No. Sho 62-27482 and Japanese Patent Application Laid-open No. Hei 6-157033 disclose production of a metal oxide thin film by the sol/gel method in which a starting material is applied onto a substrate and then the substrate is exposed to an atmosphere containing a water vapor, followed by a heat treatment. The exposure to water vapor in this method may not be effective compared with the known hydrolysis by moisture present in an ambient atmosphere when the substrate is left to stand in the ambient atmosphere. Particularly, if the thickness of the film to be applied exceeds about 0.5 xcexcm, the penetration of water vapor into the gel film is poor, so that the degree of hydrolysis will be different between the surface and the inside of the film, producing adverse effects on the film quality after the calcination. Also, in view of the productivity, it is required that the sol is stable against hydrolysis for a long period of time before its application onto a substrate. However, if the sol is highly stable against hydrolysis, a simple introduction of water vapor cannot produce sufficient hydrolysis even if the substrate is left in that state for a long period of time.
Meanwhile, a research is conducted in which a metal oxide material is applied to functional devices by utilizing the various properties of the metal oxide. For example, (Pb,La)(Zr,Ti)O3 (hereafter referred to as PLZT) has a crystal structure of perovskite type and is known to have a large secondary electrooptical effect (Kerr effect), so that PLZT can be applied to optical shutters, optical modulators, ferroelectric memories, and others. For example, by arranging PLZT fibers in an array, an optical shutter device capable of processing a plurality of beams in parallel can be realized. Also, Pb(Zr,Ti)O3 (hereafter referred to as PZT) has a crystal structure of perovskite type and is known to have a large piezoelectric effect, so that PZT can be applied to actuators and others.
Conventionally, in applying such a metal oxide to the above-mentioned functional devices, the metal oxide is first molded into a fine shape such as a fiber shape with a diameter of 5 to 100 xcexcm, followed by arranging the obtained molded products into a desired array. For example, in producing a PLZT fiber array, gel fibers are spun from a concentrated sol and arranged on a substrate to fabricate the fiber array. It is believed that the fiber array obtained by the sol/gel method can have a higher density for improving the efficiency by reducing the diameter of the fiber and arranging the fibers with a shorter pitch, as compared with those fabricated by a bulk-type machanical process.
However, the above technique requires a work of arranging the molded products, which has been formed in a fine shape, in an array on a substrate, so that the handling of the PLZT fiber whose material strength is not so large as a general glass fiber is difficult and it requires a careful attention. Therefore, in order to densify the fibers on the substrate by reducing the diameter of the fibers and arranging them with a shorter pitch, the work of arranging individual fibers becomes more difficult as the diameter of the fibers is reduced and the pitch is made shorter. This leads to increased costs due to poorer productivity caused by breakage of the fibers and the like.
Another method for producing functional devices by molding a metal oxide into a fine shape is reported in which a sol is applied onto a substrate material to obtain a gel film and then giving a fine convex-concave shape to the gel by stamping in fabricating a thin film by the sol/gel method (Kinki University; Tsutomu Minarni and Noboru Toge; HYBRIDS, Vol. 7, No. 5, pp. 15-21 (1994)). However, by such a method, the size to which the convex and concave shape can be given is limited to a degree of submicron order and, moreover, the shape that can be given is a simple convex and concave shape, so that it is extremely difficult to apply such a method to functional devices such as optical shutter devices and actuators.
The present invention has been made in view of the above circumstances and the purpose thereof is to solve the above-mentioned problems of the prior art methods of producing a metal oxide thin film by the sol/gel method and to provide a method for production of a highly dense and high-quality metal oxide thin film and a method for production of a functional device in which metal oxides having a fine shape are arranged in an array, with good productivity and at low costs.
The present invention relates to a method for production of a metal oxide thin film and a method for production of an arrayed metal oxide functional device, comprising the steps of:
preparing a sol containing a material and a solvent;
gelating the sol to give a gel member;
processing the gel member under a pressure of not less than 0.2 MPa at 100 to 400xc2x0 C. in a water-vapor-containing atmosphere; and
conducting a heat treatment on the water-vapor-processed gel member at 200 to 400xc2x0 C.